The key components of a hard disk drive are a magnetic disk and a magnetic head, which is typically separated from the magnetic disk by a small gap. The gap is created by the magnetic head remaining relatively stationary while the magnetic disk rotates on a spindle. The rotation of the magnetic disk generates a thin film of air known as an "air bearing" over the surface of the disk that supports the magnetic head, which is essentially flying over the surface of the magnetic disk.
The recording density of the magnetic disk is strongly influenced by the gap or "fly height" between the disk and the magnetic head. Because decreasing the fly height of the magnetic head increases the recording density of the magnetic disk, the magnetic disk should have a very smooth surface so the magnetic head can fly very close to the surface. Magnetic disks with protrusions related to defects or contamination that exceed the fly height of the magnetic head must be eliminated. An impact between the magnetic head and a protrusion can cause undesirable effects, such as a hard drive crash, formation of wear debris, unusable recording area, and a thermal spike in a magnetoresistive head. In order to ensure smooth surface conditions of the magnetic disk in terms of protrusions, glide tests are widely employed by the hard disk industry.
The basic operation of the glide test is to fly a test head, i.e., a glide head, at a height related to the fly height and margin requirements of the magnetic head, and to sense any contact between the glide head and defects on the surface of the magnetic disk. If the glide head contacts a defect, the disk is rejected. The term "glide head" as used in the present disclosure indicates a head used in a magnetic disk testing system as distinguished from the term "magnetic head," which is used in general for a read-write head.
The contact detection is typically accomplished with a piezoelectric (PZT) sensor or an acoustic emission (AE) sensor. The PZT sensor, as is well known to those skilled in the art, uses a piezoelectric crystal to convert mechanical energy into an electrical signal. Thus, a PZT sensor converts the mechanical energy generated by the glide head contacting a defect into an electrical signal that can be used to indicate the size and location of the defect.
An AE sensor uses a sensing technique similar to a PZT sensor. The difference is the mounting position and configuration of the sensor. The sensing material in an AE sensor is typically a PZT (PbZr0.sub.3 -PbTi0.sub.3) ceramic that has a piezoelectric effect and which is housed in a metal container and mounted close to the head/slider suspension. Both the PZT and AE sensors give electrical signals excited by acoustic vibration. For more information related to the aforementioned sensing technology see U.S. Pat. Nos. 5,423,207 and 4,532,802.
The magnetic disk may also be tested for defects sing non-contact methods such as a magneto-resistive (MR) head, a laser, or an optical tester. For more information related to MR technology see U.S. Pat. No. 5,527,110 ("the '110 patent"), and see U.S. Pat. No. 5,550,696 ("the '696 patent") for a method to calculate a protrusion height (in this case, a laser bump) based on a diffracted laser beam detected by a linear photo-detector array. An optical tester optically scans the magnetic disk for defects. The detection is usually performed by comparing the light reflected from a defect with the light reflected from an area of the disk that does not have defects. The optical tester is calibrated in such a way that a rejection of a magnetic disk occurs when the height of a defect is above a desired threshold.
Another important parameter of the magnetic disk is how low a head can fly without contacting the disk surface, known as the avalanche point. The avalanche point is different from the fly height of the head. While the fly height is usually determined by extrinsic defects, such as contamination, the avalanche point is determined by intrinsic surface design. The avalanche point is defined as the fly height at which the lowest part of the head starts to contact the disk surface. For example, the landing zone of a magnetic disk, which is usually textured to prevent excessive friction, has a high avalanche point due to the additional surface roughness created by the texturing. The data zone, however, has a smoother surface because there is no need to reduce friction. Consequently, a glide head can fly lower over the data zone than the landing zone, and thus, the data zone has a relatively lower avalanche point. The avalanche point is a useful indication of the surface finish and gives an absolute fly height below which flying is not possible without contacting the disk.
While magnetic disks are ideally flat and smooth, in practice there is typically an amount of disk waviness and runout. Disk waviness causes the effective height of a bump to vary relative to the mean disk surface. The variance is caused by the fact that the bump is defined by small area. The small area containing the single bump, however, may be at the peak of a wave or at the valley of a wave. If the waviness of the disk surface has a wavelength that is less than a longitudinal dimension of the glide head, the glide head cannot follow the disk surface. Consequently, the amplitude of the waviness must be added to the protrusion height. A typical amplitude of the waviness of a disk is 20 to 60 nm (nanometers) and has a wavelength that is typically smaller than the length of a conventional glide head.
Disk runout is a deviation from a level surface and is caused by improper clamping of the disk, for example. The runout effect typically creates a variation from a level surface over an area of the disk that is much greater than the length of the glide head. Disk runout causes acceleration of the glide head which induces fly height fluctuations. A typical disk runout is approximately 2 to 10 .mu.m (micrometers).
Where the avalanche point is compared to the protrusion height, the avalanche point is typically 25 to 50 .ANG. higher than the height of the protrusion because the protrusion height is locally measured and does not reflect the waviness or runout effect. The difference between avalanche point and protrusion height is more pronounced when the measurement is carried out close to the clamping area of the magnetic disk, which increases disk runout.
To accurately test a magnetic disk with a glide head, it is important for the glide head to be calibrated so that the glide height at which the test is carried out is known. Calibration ensures that the threshold for defects is at the appropriate height. A conventional method of calibration is performed by flying a glide head over a glass disk on a fly height tester. The fly height tester operates by passing a beam of light through the glass disk. The interference pattern of light reflected off the glide head and light reflected off the surface of the glass disk is used to determine the distance between the disk surface and the glide head. This procedure is performed for a number of different linear velocities of the glass disk to establish a linear velocity versus fly height relationship for that particular glide head.
The linear velocity versus fly height relationship is then used to determine the linear velocity at which to fly the glide head over production disks on a glide tester. Thus, the proper linear velocity can be used to achieve the desired glide height to test for defects on a production disk that are higher than the glide height. This procedure is done for each individual glide head because each glide head has different flying characteristics.
There are several drawbacks to the use of a fly height tester for calibration of a glide head. For example, the fly height tester uses a glass disk, which may have different characteristics than a production memory disk, including differences in surface texture, waviness and runout. Changes in surface texture will cause a change in the flying characteristics of the glide head and, thus, the fly height of the glide head may be different when the glide head flies over a production disk. The difference in waviness and runout between a glass disk and a production magnetic disk may additionally affect the accuracy of the calibration.
In addition, the fly height tester measures the fly height of the glide head at a limited number of points, i.e., only at the points where the light is incident on the glide head. Because the flying attitude of a glide head is not flat, i.e., the leading edge of the glide head is flying higher than the trailing edge, portions of the glide head may actually be lower than the points on the glide head being illuminated by light. Thus, the actual fly height of the lowest point on the glide head may be lower than the measured fly height.
Further problems encountered with using a fly height tester for calibration arise from the fact that the glide head is first installed on the fly height tester to determine fly height. The glide head is then removed from the fly height tester and installed on the glide tester to test production disks. Installing the glide head on two separate systems is time consuming and thus results in a loss of productivity. In addition, the glide head may be installed slightly differently on the glide tester than on the fly height tester. Differences in the installation on the two devices may cause differences in the flying characteristics of the glide head, including skew and mount flatness, thereby resulting in an inaccurate calibration of the glide head. Moreover, the Z height, which is the height between the disk surface and the suspension arm upon which the glide head is mounted, may differ between the fly height tester and the glide tester. Variances in Z height can also adversely affect the repeatability of the flying characteristics of the glide head from the fly height tester to the glide tester. A calibration disk as shown in U.S. Pat. No. 5,528,922 ("the '922 patent"), may be used to calibrate a glide head. The '922 patent uses laser bumps on a calibration disk as a height reference. Specifically, the '922 patent uses a laser beam to create crater-shaped bumps on a calibration disk to emulate undesirable asperities on a production disk and then uses the calibration disk to calibrate a glide head. While the '922 patent addresses the importance of the diameter of a laser bump to emulate a realistic defect found on a magnetic disk, there is, however, no consideration of other important aspects. For example, the '922 patent fails to consider laser bump wear, which prevents consistent calibrations and reduces the life of the calibration disk. Further, the '922 patent fails to account for disk waviness, disk runout, bump height distribution, or fly height distributions of glide heads, all of which may seriously affect the accuracy of the calibration of the glide head.